MPs will return to the green benches knowing that all outcomes are possible over the next six months.
Photograph: David Levene/The Guardian

Asked how chaotic the coming months could be in British politics, even battle-hardened veterans from both main parties struggle to find the words. “It could be utterly ghastly, with a complete breakdown in party discipline,” says one former Tory cabinet minister. “It is unprecedented in my 30 years.”

A Labour MP is similarly apocalyptic: “This is probably the most dangerous, existentially dangerous, period for the Labour party since 1981. It’s not clear that the party will survive this time.”

The intrigue has already begun. Potential Tory leadership candidates have been dining with donors. Conservative MPs have been organising a “Stop Boris” campaign. Labour MPs have been discussing when and how to split. And pro-Brexit MPs have been stirring up constituency trouble for their Remain-supporting colleagues.

Chuck in the few dozen crates of Pol Roger champagne that come with the conference season, sprinkle on the threat that some MPs could have their seats abolished altogether, and you have a potent recipe for months of political mayhem. The only thing that unites the various factions plotting across the political spectrum is complete uncertainty over how the drama will play out.

All these volatile ingredients will be thrown together in Westminster this week when MPs return to the Commons. At the heart of the febrility is Britain’s future relationship with the European Union. After more than two years of paralysis, Theresa May will have to find a way of signing off on a withdrawal agreement with Brussels and gaining sufficient support for a future relationship with the bloc.

MPs return to the green benches knowing that all outcomes are possible over the next six months – from a swift leadership challenge to May, to the collapse of Brexit talks, to the collapse of the government, to the implosion of the Labour party, to the crowning of Jeremy Corbyn as prime minister. Yet just as likely, suggest both supporters and some critics of May, is the notion that she will somehow dodge, duck and delay her way to next March, having secured support from Brussels and parliament for a Brexit deal.

“Don’t underestimate Theresa – not because she is brilliant, but because people have worked themselves up into a froth,” says one veteran MP. “If she comes up with a deal and it is clear we leave the union in March, people’s attitude will change.”

In the middle of it all comes a conference season in which the likes of Boris Johnson and David Davis will drum up opposition to the prime minister’s Brexit plans. It could also pitch Labour into another round of infighting over party rules and its own Brexit position, with constant talk of splits and breakaway parties.

Just to add extra spice to proceedings, this week will also see the publication of the review of parliamentary boundaries, which reduces the number of constituencies from 650 to 600. The reform hits Labour hardest, but will also cost the Conservatives seats. Some ministers are already convinced that the reform will become a lightning rod for rebels and have told the whips to ditch it. A vote on the measure is due this autumn, just when May needs all the support she can muster.

“The timing is awful, just when everything is fluid over Brexit,” said a frontbencher. “They are firefighting on other fronts and they really don’t need this.”

The final coup de théâtre will be the meaningful vote on the final Brexit deal. As suits such a period of political turmoil, it could come down to whether enough Labour MPs are willing to back whatever deal is on offer from May, in effect saving her leadership.

It will be a huge moment for parliament and an agonising one for MPs across the Commons.

A new Tory MP, who has been horrified at the state of his party since his election, has the best and most pithy advice to colleagues making their way back to Westminster this weekend: “Strap in, would be my take on it.”

Conservatives

Theresa May’s future as leader will be in doubt throughout the next six months. She has to steer an impossibly narrow course between the demands of Brussels, her cabinet and an unpredictable parliament in order to still be in post by the end of March, when Britain leaves the EU.

May’s problem is not just that several senior Tories have openly declared that they will vote down the proposed Brexit agreement she outlined to the cabinet at her Chequers retreat in July. It is that in securing a withdrawal agreement and coming up with a workable future relationship, she will probably need to make further concessions. The uncertainty means final talks with Brussels are already set to slip from October to November. Even May loyalists believe there is no obvious Commons majority for the Chequers model.

At any moment, MPs could trigger a leadership election. That is most likely to come from Brexiters determined to remove her before she can force through a deal they can’t stomach. “If she presses ahead with Chequers (or, more likely, a watered-down version), it’s hard to see how we avoid a decisive showdown before Christmas,” said one pro-Brexit MP.

Brexiters have a figurehead in Boris Johnson, though he is unlikely to wield the knife. “He’ll leave the hard work to someone else,” says a former minister.

Between now and Brexit day, May also has to pass lots of preparatory legislation, any part of which could be hijacked by Leave or Remain MPs fighting their cause. Party discipline is not helped by poisonous relations between some pro and anti-Brexit MPs. Several Tories cite Jacob Rees-Mogg writing to local Conservative associations, urging them to “chuck Chequers”. Even in the dog-eat-dog world of Westminster, that was regarded as overstepping. “You just don’t do that,” says a senior minister.

At conference, the gloves will be off. Johnson, David Davis and Rees-Mogg could make life intolerable for May, pressing for their version of Brexit.

Most MPs expect a Commons vote on the final Brexit deal by the end of the year. Should May lose that, few agree on what would follow: a leadership challenge, an election or a second referendum.

Labour

Well before MPs get anywhere near voting on Brexit issues, the Labour party will have faced some major internal battles. In fact, the first big clash will come on Tuesday, the day that MPs return to Westminster.

After a summer that was largely dominated by a row over the party’s handling of antisemitism claims, its ruling national executive committee will meet to decide what action to take. Most MPs want the party to adopt the full definition of antisemitism set out by the International Holocaust Remembrance Alliance and its related examples. The leadership is concerned that one of the examples could be regarded as damaging free speech, so a fudge is expected.

From that struggle, the party will then barrel straight into its own dispute over Brexit. Its ambiguous Brexit position will come under intolerable strain. Shadow Brexit secretary Keir Starmer’s team remain convinced that May’s deal can be voted down without provoking a “no-deal” outcome, but other Labour figures are not so sure. There will be pressure at the party’s conference from Labour members wanting a second referendum.

Meanwhile, the Labour conference is also likely to see further attempts to change party rules and hand more power to the party’s left. Some attempts will be killed off, but not without a lot of bad blood and more internal strife.

Even when Brexit is done with, the party battle could just be beginning. Several Labour MPs believe that the key moment for the party’s long-term future will come in the spring, after Brexit has happened.

“I don’t see any mass departure before April or May next year,” said one MP involved in the split discussions. “People feel that we do have a duty to the country to see through the Brexit process, and we won’t be forgiven if we distract from that.

Other parties

For Vince Cable and the Liberal Democrats, the next few weeks could have repercussions that will be felt for years to come.

The party has failed to make any major progress in the polls since last year’s general election and there are frustrations among members over the lack of impact that Cable has had since becoming leader following Tim Farron’s resignation in June 2017.

This week, the leader will make a speech setting out proposals designed to revitalise the Lib Dems by allowing non-MPs to run as party leader – and he will also set out his own future plans.

Cable, 75, is expected to suggest that he will not fight an election should it come in 2022.

In the meantime, the party has adopted an anti-Brexit stance and is backing calls for a second referendum that could become significant when May’s deal comes to parliament.

The unpredictability of the autumn means that other parties – and even newly formed ones – could end up playing a major role in the months ahead.

May’s need to come up with a compromise to secure a Brexit deal, with more compromises likely, has opened up an opportunity for a Ukip-style party.

While Ukip has struggled to make an impression since the referendum, Nigel Farage signalled his return to frontline politics over the summer to fight May’s Brexit plan.

Former Ukip donors have talked about creating a new movement on the right to argue for a hard Brexit. Any sense that May is “betraying” the vote to leave the EU could present an opportunity for the Tories to be attacked from the right again.

Meanwhile, the space in the political centre ground has become so great that plans for new centrist parties and movements could finally gain some traction.

United for Change, set up by former Labour donor and businessman Simon Franks, is having some success recruiting members. However, it has already been beset with internal problems, with some staff leaving to set up an alternative group.

The political dynamics could change again should sitting MPs sign up or start their own project.